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SoloMail is a WordPress plugin designed to email an HTML-formatted copy of a single post to site subscribers or a mailing list. There are quite a few solutions out there designed to send batch digests of recent posts to subscribers on a scheduled basis. SoloMail solves a different problem – you may want to cherry pick just certain posts to go out via email, and you may want the ability to “send now.”
The subscriber list can EITHER be the group of all registered users on a site OR a single address, such as a mailing list. Though you can use SoloMail either way, I recommend sending through a proper mailing list, which provides advantages such as handling and unsubscribing bounced/dead addresses, and metered sending for large lists.
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SoloMail

Emails an HTML-formatted copy of a single post (not batch/digests) to site subscribers.

SoloMail vs WP Mail SMTP by WPForms – The Most Popular SMTP and Email Log Plugin